This two lane road is part of State Highway 3 along one side of the gorge. It's quite susceptible to landslides that close it occasionally.
Beginning as a surveyor’s line, by 1872 a “road” existed but was a perilous trip for coaches. Travellers had to cross the river by punt or take an aerial trip using a cageway, pulley and wire system suspended high above the river.
On the other side of the Manawatu River is a train track.
The railway through the gorge was contracted for in 1886. Men worked in large gangs shovelling metal winched up from the river. The line was completed in 1891 and the first train over it carried 1500 passengers in eighteen double carriages.
At least, there's some interesting scenery to while away the 8 kilometers that seem like a lot more :)
Info from: http://www.doc.govt.nz/parks-and-recreation/places-to-go/manawatu-whanganui/places/manawatu-gorge-scenic-reserve/